In the use of progressing cavity motors of the fluid driven type for driving a rotary drill, utilizing drilling fluid to drive the motor rotor and to flush cuttings from the bore hole, severe problems of wear occur due to the erosive nature of the drilling fluid and the heat of friction, and in the case of wells, the geothermal conditions. Accordingly, efforts have been made to resolve such problems.
Reference is made to U.S. Pat. 4,029,368, granted June 14, 1977, to me and John E. Tschirky, for "Radial Bearings", which contains a general discussion of some of these problems and provides improved radial bearing means for such structures to handle the periodic transverse forces caused by conversion of the eccentric motion of the motor rotor to rotary motion of the drive shaft, which is generally accomplished by a connecting rod assembly comprising a rod having universal joints at its ends connected with the motor rotor and with the drill drive shaft.
Typically, the universal joints are lubricated by a quantity of grease contained in the joints by a flexible cover or boot which is strapped about the universal joint and enables the relative motion to occur. When such boots are destroyed by external pressure extruding the boot into the voids between the working parts of the universal joint, or expansion of the boots caused by gassing or expansion of the grease due to the heat generated by the coupling, the universal joint is rapidly destroyed by the erosive effect of the fluid driving the motor.
Efforts have heretofore been directed toward alleviating the problem of destruction of the boot around universal joints of in-hole motor-drill connecting rods by providing means which are supposed to equalize the pressure of the lubricant inside the universal joints with the pressure of fluid in the housing.